Two types of gneisses associated with eclogite at Shuanghe in the Dabie terrane: carbon isotope, zircon U–Pb dating and oxygen isotope

There are two types of gneisses, biotite paragneiss and granitic orthogneiss, to be closely associated with UHP eclogite at Shuanghe in the Dabie terrane. Both concentration and isotope composition of bulk carbon in apatite and host gneisses were determined by the EA-MS online technique. Structural carbonate within the apatite was detected by the XRD and FTIR techniques. Significant ¹³C-depletion was observed in the apatite with δ¹³C values of −28.6‰ to −22.3‰ and the carbon concentrations of 0.70–4.98 wt.% CO₂ despite a large variation in δ¹⁸O from −4.3‰ to +10.6‰ for these gneisses. There is significant heterogeneity in both δ¹³C and δ¹⁸O within the gneisses on the scale of several tens meters, pointing to the presence of secondary processes after the UHP metamorphism. Considerable amounts of carbonate carbon occur in some of the gneisses that were also depleted in ¹³C primarily, but subjected to overprint of ¹³C-rich CO₂-bearing fluid after the UHP metamorphism. The ¹³C-depleted carbon in the gneisses is interpreted to be inherited from their precursors that suffered meteoric–hydrothermal alteration before plate subduction. Both low δ¹³C values and structural carbonate in the apatite suggest the presence of ¹³C-poor CO₂ in the UHP metamorphic fluid. The ¹³C-poor CO₂ is undoubtedly derived from oxidation of organic matter in the subsurface fluid during the prograde UHP metamorphism.

Zircons from two samples of the granitic orthogneiss exhibit low δ¹⁸O values of −4.1‰ to −1.1‰, demonstrating that its protolith was significantly depleted in ¹⁸O prior to magma crystallization. U–Pb discordia datings for the ¹⁸O-depleted zircons yield Neoproterozoic ages of 724–768 Ma for the protolith of the granitic orthogneiss, consistent with protolith ages of most eclogites and orthogneisses from the other regions in the Dabie–Sulu orogen. Therefore, the meteoric–hydrothermal alteration is directly dated to occur at mid-Neoproterozoic, and may be correlated with the Rodinia supercontinental breakup and the snowball Earth event. It is thus deduced that the igneous protolith of the granitic orthogneiss and some eclogites would intrude into the older sequences composing the sedimentary protoliths of the biotite paragneiss and some eclogites along the northern margin of the Yangtze plate at mid-Neoproterozoic, and drove local meteoric–hydrothermal circulation systems in which both ¹³C- and ¹⁸O-depleted fluid interacted with the protoliths of these UHP rocks now exposed in the Dabie terrane.